Its said that pygmyies don't need UVB...so if that's true...I guess they can go forever without it. (I always gave mine some UVB...so you need to hear it from someone who hasn't.)
As for veileds and panthers and other "sun-loving" chameleons...there's no definite length of time. It also depends on whether they are getting D3 from supplements or from something that the insects ate IMHO.
I know very little about Pygmy chameleons but I do know Steve Irwin said they live on the forest floor amongst the leaf litter.
Wouldn't it stand to reason that if such a tiny creature spent its life rustling through leaves under the cover of a lushfully large forest canopy that very little sunlight would be greeting them with a hello? I wonder how evolution has worked on their tiny little bodies to make them efficient ray catchers? Is it possible they are getting all they need from the ambient light of the forest floor?
I think this thread could be on to something but I am always a geek for science.